Various methods for modifying the tensile, dimensional heat stability (residual shrinkage), and dyeability characteristics of drawn crystalline filaments of poly(ethylene terephthalate) are known which involve heating at a high temperature while maintaining the filaments under a controlled tension or length. For instance, British Patent Specification No. 735,171 to ICI discloses that the dyeability of yarns of poly(ethylene terephthalate) can be enhanced and shrinkage decreased by a heat treatment at temperatures on the order of 210.degree.-230.degree. C. while the yarn may or may not be allowed to shrink. U.S. Pat. No. 3,044,250 to Hebeler discloses a process for heating drawn filaments under drawing tension to improve their load-bearing properties relative to those of certain natural fibers and to improve their resistance to shrinking.
U.S. Pat. No. 3,816,486 to Vail discloses a two-stage drawing process for poly(ethylene terephthalate) filaments which can be followed by relaxation or annealing. Disclosed annealing temperatures range from 180.degree.-240.degree. C. Art such as this generally does not recognize any criticality associated with the manner for heating the filaments with regard to the resulting annealed filament properties. The presently preferred heating method for annealing drawn filaments in commercial polyester staple production is the use of heated metal rolls.
U.S. Pat. No. 3,452,132 to Pitzl discloses a process for simultaneously drawing and annealing polyester textile yarn wherein steam is jetted on the filaments in an enclosure with sufficient velocity to open the bundle and to raise the temperature of the filaments substantially instantaneously to at least above their second order transition temperature and thereby initiate drawing with simultaneous annealing. Steam at superatmospheric pressure is used as the fluid for the high velocity jet because of the effectiveness of the condensing steam as a heat transfer agent. The steam pressure is said not to be critical and no benefits are indicated for increasing the yarn temperature to above about 180.degree. C. Such a process is not compatible for drawing and annealing large bundles of filaments such as employed for tows in the manufacture of polyester staple.
Recent advances in the synthetic staple fiber industry, such as open end spinning and higher speed looms for weaving, have created an increased demand for stronger staple fibers, but without compromising other properties such as dyeability and especially a low residual shrinkage which is the main reason for subjecting polyester filamentary tows to annealing process.
The combined objectives of high dyeability and high tensile properties remain somewhat irreconcilable in commercial hot-roll-annealing processes. An increase in one of these properties generally must come about through some compromise in the other. Similarly opposed interactions are also found when attempting to optimize properties such as low shrinkage, crimpability, and a low amount of surface cyclic trimer. Consequently, considerable incentive remains for discovering a commercially feasible process which can provide an overall better combination of such properties, i.e., one which involves less sacrifice in one or more individual properties to improve another.
An object of this invention is a process for annealing a tow of drawn filaments of poly(ethylene terephthalate) to provide an improved balance of filament properties including strength, dyeability, and shrinkage, and/or crimpability, and/or low surface cyclic trimer deposits. Another object is the improved products made thereby. Still another object of the invention is annealed crimped filaments of poly(ethylene terephthalate) having a novel unexpected combination of fine structure and improved filament properties.
These and other objects are provided by this invention.